Some of air-conditioning apparatuses known in related art, such as multi-air-conditioning units for buildings, have a refrigerant circuit in which, for example, an outdoor unit to serve as a heat source unit disposed outside a building, and an indoor unit disposed inside the building are connected by pipes. Refrigerant circulates in the refrigerant circuit, and air is heated or cooled by utilizing the rejection or absorption of heat by the refrigerant, thus heating or cooling the air-conditioning target space. In recent years, air-conditioning apparatuses employing fluorocarbon refrigerants with low global warming potentials, such as an R32 refrigerant, have been considered for use in multi-air-conditioning units for buildings.
As opposed to an R410A refrigerant widely used in conventional air-conditioning apparatuses such as multi-air-conditioning units for buildings, an R32 refrigerant is characterized by its high temperature at discharge from the compressor. The high discharge temperature causes problems such as degradation of the refrigerating machine oil, leading to damage to the compressor. Accordingly, to lower the discharge temperature of the compressor, the rotation speed of the compressor needs to be lowered to reduce the compression ratio. This makes it impossible to increase the rotation speed of the compressor, leading to insufficient cooling capacity or insufficient heating capacity. The following approach is being proposed to address this problem. According to this approach, refrigerant in a gas-liquid two-phase state is injected into a medium-pressure chamber, which attains a medium pressure during the compression process of the compressor, thus lowering the discharge temperature of the compressor while allowing for an increase in the rotation speed of the compressor (see, for example, Patent Literature 1).